A Mexican retablo, or ex-voto painting.

There are many different ways to think about folk art. In fact, there is no one definition of folk art. In collecting and displaying folk art, the museum considers various concepts.

Generally, folk art is ART that:

• May be decorative or utilitarian
• May be used every day or reserved for high ceremonies
• Is handmade; it may include handmade elements, as well as new, synthetic, or recycled components
• May be made for use within a community of practice or it may be produced for sale as a form of income and empowerment
• May be learned formally or informally; folk art may also be self-taught
• May include intangible forms of expressive culture like dance, song, poetry, and foodways
• Is traditional; it reflects shared cultural aesthetics and social issues. It is recognized that, as traditions are dynamic, traditional folk art may change over time and may include innovations in tradition.
• Is of, by, and for the people; all people, inclusive of class, status, culture, community, ethnicity, gender, and religion

http://www.internationalfolkart.org/learn/what-is-folk-art.html

The CCAD Historic Art Book Collection has a number of items relating traditional folk design & craft: 

Batiks de Java, n.d. (1920's)
Broderies chinoises [Chinese embroidery], (1920?) 
Le costume historique, [Historic costume],  A. Racinet (6 vols.), 1888
Le Costume historique, v. 1-2, (our "mini" Racinet), 1890
Costume of the hereditary states of the House of Austria, 1804
Costume of Hindostan, 1804 
Costume of the Russian Empire, 1803
Costume of Turkey, 1804
Decorations polychromes, 18? [Various decorative motifs from around the world]
Decoration primitive, Afrique : quarante-deux planches, [Primitive Decoration of Africa], 1922
Décoration primitive Océanie : quarante-deux planches [Primitive Decoration of Oceania], n.d. (1920's)
Étoffes byzantines, coptes, romaines, etc. du IVe au Xe siècle [Byzantine, Coptic, Roman rugs from the 4th - 10th centuries])
Kultur und industrie südamerikanischer völker [Culture and industry of South American peoples], 1889-90
L'ornement polychrome...motifs de tous les styles, art ancien et asiatique, Moyen Age, Renaissance, XVIIe et XVIIIe sièce, [The polychrome ornamental patterns ... all styles, old and Asian art, medieval, Renaissance, seventeenth and eighteenth centuries],188-?
Der Ornamentenschatz; ein Musterbuch stilvoller Ornamente aus allen Kunstepochen [The treasure ornaments, a pattern book of stylish ornaments from all periods of art] ,1887
Pennsylvania German illuminated manuscripts, 1937
Religious art in Finland during the Middle Ages, 1921
Tapisseries et étoffes coptes [Coptic Tapestry and Rugs], 191-?